Women with primary vasospastic syndrome (VS) but otherwise healthy exhibit a functional disorder of vascular regulation (main symptom: cold extremities), and often suffer from difficulties initiating sleep (DIS). Diverse studies have shown a close association between distal vasodilatation before lights off and a rapid onset of sleep. Therefore, we hypothesized that DIS in women with VS could be due to a reduced heat loss capacity in the evening i.e. subjects are physiologically not ready for sleep. The aim of the study was to elucidate whether women having both VS and DIS (WVD) or not (controls, CON) show different circadian characteristics (e.g. phase delay of the circadian thermoregulatory system with respect to the sleep-wake cycle). Healthy young women (N=9 WVD and N=9 CON) completed a 40-h constant routine protocol (CR, adjusted to habitual bedtime), before and after a 8-h sleep episode. Skin temperatures (off-line calculated as distal-proximal skin temperature gradient, DPG), and core body temperature (CBT, rectal) were continuously recorded. Half-hourly saliva samples were collected for melatonin assay and subjective sleepiness was assessed on the Karolinska Sleepiness Scale (KSS). In comparison to CON, WVD showed no differences in habitual bed times, but a 1 h circadian phase delay of dim-light-melatonin-onset (h after lights on: WVD 14.6 ± 0.3h; CON 13.5 ± 0.2h; p=0.01). Similar phase shifts were observed in CBT, DPG, and KSS ratings. In conclusion, WVD exhibit a phase delay of the endogenous circadian system with respect to their habitual sleep-wake cycle, which could be a cause of DIS.